The invention relates to a method and an apparatus for pulling monocrystals from a melt under vacuum or under inert gas with an apparatus having a crucible which is disposed in a vacuum chamber and heated by the thermal radiation of heating elements. A pulling device is provided above the melt situated in the crucible, by which the monocrystal can be pulled upward out of the melt surface. A feeding device is provided whose filler opening terminates above the crucible and through which the charge material can be added to the crucible by a conveyor during the pulling process. Measuring sensors continually provide the characteristic parameters of the crystal pulling process into a controller which controls the conveyor.
Canadian Patent No. 997,255 discloses an apparatus for pulling crystals out of a melt, having an optical device for monitoring and/or controlling the diameter of the growing portion of the crystal. The optical device contains photoelectric cells whose signals are entered via amplifiers into a PID controller, which in turn is connected to a high-frequency generator which serves as a power control and which also supplies the current for the heating coil placed around the melting crucible. The level of the molten bath is used in this device as a factor for controlling the heating current.
Also known is a method and a system for monitoring and controlling crystal growing processes (DD 253,437) in which the temperature pattern as well as temperature variations in the vicinity of the solidification front are measured so that a corresponding correction can be made after comparison with a temperature pattern that is typical for the crystal growing process involved. The thermal radiation emitted by a crystal growing out of a melt and that emitted by the melt itself are sensed by means of a linear or broad-surface arrangement of photosensors consisting of at least two spatially separate individual elements, and difference signals are formed whose magnitude is proportional to the temperature change in the defined area, thereby detecting temperature patterns which are constantly brought into agreement with the predefined temperature pattern.
A computer permits the correction of the nonlinear relationship between temperature and the magnitude of the signal, so that an actual temperature corresponds to the value of each signal.
U.S. Pat. No. 4,832,496 discloses a measuring method and a measuring system for the diameter of monocrystals being pulled from a crucible, in which use is made of an optical detection of the brightness profile at the point of transition from the melt to the monocrystal, and of a reading of the position of the brightness profile in relation to a point of reference on a display. The images of the melt/monocrystal transition or its intensity signals are connected to an evaluation circuit for measuring the diameter, and by means of these produce an effect on the pulling parameters, i.e., for example, quantitatively the level of the melt in the crucible, or qualitatively the melting of the starting material which is in granular or lump form.
These known evaluating circuits, however, are not satisfactory in practice, since the pulling process is extremely complex overall, and also depends on a series of effects and data determined by experience, which cannot be processed correctly or fast enough by the former control circuits, since they do not correspond to numerical equations or classical mathematical logic or often are based on ambiguous readings.